1. Field of the Invention
The present invention relates to a device for transferring semiconductor wafers from cassettes to a boat and from the boat to the cassettes.
2. Description of the Related Art
In the case of the semiconductor elements manufacturing apparatus, a plurality of wafers mounted on a boat made of quartz are loaded into the heat process section and a heat diffusion process is applied there to the wafers as a unit.
Prior to applying this heat diffusion process to the wafers, the wafers housed in a cassette must be transferred from the cassette onto a boat by means of a transfer device for the wafers.
The transfer device for the wafers has a pair of chuck members to carry out this transferring of wafers. A plurality of grooves parallel to one another are formed on those faces of the chuck members which are opposed to each other, at the same pitch as the grooves of the cassette.
The wafers housed in the cassette are lifted to a certain level above the cassette by means of the lifter means, held between the chuck members, and carried to the boat. The wafers are loaded on the boat in those grooves which are formed on the inner face of the boat at a certain pitch interval.
This transferring or loading of wafers is repeated relative to plural cassettes placed on a cassette table to successively transfer the wafers from the cassettes to the boat.
Up to twenty-five pieces of wafers are usually housed in the cassette at a pitch or spacing interval of 3/16 inches and mounted on the boat at the same pitch interval by means of the transfer device for the wafers.
However, various kinds of processes are applied to the wafers, and the wafers have various sizes. Therefore, it is sometimes necessary that the wafers are transferred onto the boat at a pitch interval of 6/16 inches or 9/16 inches different from that of wafers holding grooves in the cassette and two, three or more times that of the wafers holding grooves in the cassette.
Japanese Utility Model Publication Sho 61-33443 discloses a transfer device for wafers which is capable of changing the pitch interval of the wafers mounted on the boat. As shown in FIG. 1, first grooves 14a for holding wafers and second grooves 14b for holding no wafer are formed alternately and parallel to one another on the inner face of each of the chuck members at the pitch interval of 3/16 inches.
Twenty-five, twenty and twenty-five pieces of wafers are housed in cassettes 10A, 10B and 10C, respectively, at the pitch interval of 3/16 inches, as shown in FIG. 2A. Let us assume that the wafers in each of the cassettes are transferred onto boat 12 at the pitch interval of 6/16 inches. Fixing dummy wafers 16 are previously mounted on boat 12 at both ends thereof.
Twenty-five pieces of wafers are lifted above cassette 10A by means of the lifter means (not shown) and held between chuck members 14, as shown in FIG. 2B. First and second grooves 14a and 14b are alternately formed on each of the chuck members 14. Therefore, only uneven-numbered wafers of those twenty-five pieces of wafers shown in FIG. 3A are held between chuck members 14 and transferred on boat 12 at the pitch interval of 6/16 inches, as shown in FIG. 2C (wafers WA1 shown in FIG. 2C).
Chuck members 14 are shifted from their previous position by 3/16 inches to hold the remaining even numbered wafers between them, as shown in FIG. 2C. The wafers thus held are transferred onto boat 12 at the pitch interval of 6/16 inches, as shown in FIG. 2D (wafers WA2 shown in FIG. 2D).
This transferring operation of the wafers is repeated relative to cassettes 10B and 10C, as shown in FIGS. 2D through 2E. A plurality of wafers are finally arranged on boat 12 at the pitch interval of 6/16 inches, as shown in FIG. 2F.
In the case of the conventional wafers transfer device, however, wafers W which are housed in cassette 10 at the pitch interval of 3/16 inches with no wafer missing can be transferred onto boat 12 at the pitch interval of 6/16 inches which is different from that of wafers W housed in cassette 10, but wafers W which are housed in cassette 10 with some of them missing cannot be transferred onto boat 12 at a desired pitch interval.
When wafers W which are housed in cassette 10 with some of them missing as shown in FIG. 3B are transferred onto boat 12 by means of the conventional wafer transfer device, they are also arranged on boat 12 with some of them missing as shown in FIG. 4.
Particularly when some of wafers are missing from those transferred onto boat 12 as shown in FIG. 4, those conditions under which processes are applied to wafers W become different in relation to some of wafers W. When diffusion and CVD processes are applied to wafers which are arranged onto boat 12 with some of them missing, a film formed on each of wafers W becomes different in quality and property.